sequences problem.

We have these two sequences, defined as:

Xn+1= Xn + Yn and Yn+1=Xn-Yn

and X0=Y0=1.

Find simple formulae for Xn and Yn. Don't even know how to start this... any help is welcome. I know it must be simple, but don't know how to do it. Usual difference equations I had were x smth and then I could use lambda exchange and find coefficients to solve it. This I don't know.